The present invention relates to a protective circuit apparatus. More particularly, the present invention relates to a protective circuit apparatus in which power is disconnected from an electrical device in response to the detection of current flow in excess of a predetermined level in the ground line or the detection of current flow in excess of a predetermined level in the hot line. The present invention is also particularly directed to a combination of three protective circuits in the form of a two to three wire converter circuit, a ground fault detector and interrupter circuit and an overload trip-out circuit, which may be advantageously used in combination to provide maximum protection for a user of an electrical device.
There are many industrial and domestic applications in which it is desired to provide a protective circuit which protects against certain circuit faults or abnormalities. Such abnormalities include current overloads, excessive torque demands, short circuits and current leakages to ground. It is also desirable to employ a combination of protective circuit devices which provide protection against all of these possible abnormalities.
In recent years, the accepted way of protecting against these faults is to provide the electrical device being used with a ground connection. The electrical devices protected in this way include all types of stationary electrical equipment and devices, such as major appliances, and portable electrical devices, such as electric drills, saws and other power hand tools and portable appliances. The ground wire is connected to the housing of the electrical device and it is assumed that the housing of the electrical device will not rise above ground potential. However, this assumes that there is a perfect connection of the electrical housing to ground potential. This is not always a valid assumption, as a fault, such as an open circuit condition or a high resistance condition, may occur at numerous points between the electrical device and the point of a true ground connection, such as at connections in receptacle boxes, connections made at connection points in the wiring between the main distribution box and the electrical outlet, connections in distribution box and connections to a water line or other ground connection plane.
There is a need for a circuit that will detect current flow in the ground connection above a predetermined level. Preferably, the predetermined level of ground current which will energize a disconnect circuit is relatively low. There is also a need for a protective circuit in the form of an overload trip-out circuit which will disconnect power flow to an electrical device when the current flow in the power or hot lead exceeds a predetermined level. Preferably, the predetermined level of current flow through the high or hot lead at which point the power flow to the electrical device is disconnected is selectably adjustable and relatively larger than in the case of a ground fault detector. For example, in electrical devices, such as hand tools, it may be very desirable to have the power disconnected to an electrical device, such as an electric drill, if the torque demand suddenly becomes excessive. For example, in the use of an electric drill, utilizing a wood boring bit, it is desirable that the power to the drill be disconnected if the wood bit hits a nail or other very hard object which might result in the user of the electrical device receiving a broken wrist or other injury.
There is also a need for a protective circuit system which would enable the use of the ground line current detector and interrupter circuit and the overload trip-out circuit in situations where only two socket receptacles are available, one for the high line and one for the neutral line. This is in contrast to the now normally accepted practice of installing three socket receptacles having a high line connection, neutral line connection and a ground line connection. The overall protective system would utilize a two to three wire converter circuit as disclosed in our co-pending application entitled "Two to Three Wire Converter Circuit" referred to above, as the input to the ground fault detector and interrupter circuit and overload trip-out circuit disclosed and claimed herein.